(especially in the following paragraph:” If you are arriving Monday-Friday between 10 and 16:15 you can recover your keys, in the hotel reception. The hotel is located next to the Polytechnique Cafeteria (see building 8 on this map or building 12 on this map. After hours, or during weekends and holidays the keys must be recovered at the “salle de permanence” building 20 on this map .”)

to go from Polytechnique to the LPT you have to walk (down) to the RER and to take the train to Bures-sur-Yvette or Orsay (in the direction Saint Remy)

From the Bures-sur-Yvette station you have to walk down until you cross a samll river (Yvette) on a small bridge then you turn right and cross immediately a second time the small river, then you walk straight for 3 minutes. The building has the number 210 and the conference is on the second floor.